FORGET the perfectly balanced bodies of a marble Adonis or Venus. In the world of real flesh, asymmetry rules. Hands, feet, ears, ankles, breasts – all can, and indeed do, differ in size or shape from one side of the body to another, if only by millimetres.
To our conscious selves, these small deviations from symmetry are seldom more than a source of mild irritation when trying on new shoes. But in the deeper recesses of our sexual psyche, in the barely conscious calculations we make to assess physical attractiveness, they may loom large. You see, asymmetrical flesh just isn’t sexy – or at least this is the message coming from research into the way deviations from perfect body symmetry influence success in the human mating game.
It may sound like a bizarre thing to investigate, but the biologists involved in the research couldn’t be more serious, or come with finer academic track records. To these researchers, “fluctuating asymmetries” (as the deviations are more formally known) are no mere physical flaws. Rather, they are central to understanding where some of our notions of physical attractiveness come from; to teasing apart the ingredients of beauty and asking if those ingredients are adaptive in a Darwinian sense; in short, to challenging the view that beauty is largely a cultural construct, and a male-dominated construct at that.
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In the politically correct campuses of north America, such a quest demands no small degree of commitment. “This asymmetry stuff is so far out of the mainstream it’s hard to get funding for it,” says Randy Thornhill, a behavioural ecologist at the University of New Mexico, Albuquerque, and a standard bearer in the crusade to examine beauty through the prism of Darwinism. “Looks really matter. We’re trying to find out what these looks are and how they evolved.”
The idea that body symmetry (or the lack of it) may be a crucial element took off three years ago. While studying the sexual shenanigans of the common Japanese scorpion fly, Thornhill discovered that males with the most symmetrical wings won the most mates. Similar biases were turning up in other animal studies. Anders Møller, now at the University of Copenhagen, found he could ruin male swallows’ chances of finding mates merely by making their tails less symmetric. One by one, the peafowl, zebra finch and earwig revealed a secret passion for symmetry. Even bumblebees came out of the closet, revealing a preference for flowers with even arrays of petals (see New Scientist, Science, 18 March).
Now, with several studies of human preferences under their belts, and more in the pipeline, Thornhill, Møller and others are claiming that our own species is also a sucker for symmetry. And not just for aesthetic reasons, either. Humans prefer symmetrical features in a face or body, suggest the researchers, because the symmetry advertises biological quality – fine genes, a strong immune system, a nourishing diet. Not to mention reproductive vigour. According to the latest findings, women partnered to men with pleasingly symmetrical bodies have the most orgasms, and women with symmetrical breasts are more fertile than those less evenly endowed.
Facial attraction
These claims look certain to create a stir, if not bemused incredulity, when they appear in journals later this year. But that is unlikely to deter biologists such as Thornhill and Møller who have come to expect criticism. “The issue is especially problematic in the US,” says Møller about the idea that sexual preferences for certain types of physique might be adaptive in a Darwinian sense. “But even here in Denmark I dare not discuss my findings in some seminars.”
The most ardent critics tend to be sociologists or anthropologists. But even within the ranks of biology, there are divisions. Researchers who think it is valid to apply zoological theories to humans are the most positive about the latest results. “People like Anders Møller have come up with so much data that we have to take [the idea about symmetry] seriously,” says Robin Baker, a biologist at the University of Manchester who has recently begun a study measuring the sizes of male ejaculates to see if symmetrical men produce the most sperm. But zoologists of a more traditional bent, who don’t “do people”, voice unease. “An awful lot of human behaviour is probably there for cultural reasons that may or may not be adaptive,” says Mark Kirkpatrick, a zoologist at the University of Texas, Austin.
Part of the problem is that the adaptationist view challenges the much cherished notion that people have their own idea about what makes for a beautiful body and face. Not true, argue Thornhill and his colleagues – at least not when it comes to symmetry. In lab tests, men prefer photographs of women with symmetrical facial features, and vice versa. What’s more people who score highly on tests of facial attractiveness also tend to have the most symmetrical bodies.
Thornhill and his colleague Steven Gangestad came to this conclusion by sizing up the asymmetries of some 72 student volunteers. Calipers in hand, they measured the length and breadth of each subject’s ears along with the widths of their feet, ankles, hands, wrists and elbows. For each volunteer, the researchers took the relative differences between left and right measurements and summed these differences for all seven traits. In this way the researchers could measure overall body asymmetry – a characteristic which, according to previous studies, shows some tendency to be inherited, rather than merely vary haphazardly from person to person (as do the asymmetries of isolated traits such as hands or feet).
The trends Thornhill and Gangestad found in their measurements seem seductively straightforward. Volunteers judged to have the most attractive faces had left-to-right asymmetries averaging at between 1 and 2 per cent per trait, while those judged to have the least attractive faces had much higher asymmetries, averaging out at between 5 and 7 per cent per trait. The message here, says Thornhill, is that there is “some information in the face about body symmetry”. And that information is probably symmetry itself: “People with the most symmetrical bodies also have the most symmetrical faces.”
But physical attraction is a slippery thing, and preferences expressed in psychology labs may not chime with those expressed in pick-up bars and bedrooms. Does body symmetry matter in the real world? This is where the caliper-and-questionnaire findings become more provocative. Men with the most symmetrical bodies tend to start having sex some three to four years before less symmetrical men, reported Thornhill and his colleagues last autumn in a paper that looked at the sexual histories of 122 students. On average, students with near perfect body symmetry – hands, wrists, feet, ankles, ears and elbows – also reported two to three times as many lifetime sexual partners to date as those with the most asymmetrical bodies.
A paper to appear in the journal Animal Behaviour later this year goes even further. In it, Thornhill and his colleagues claim that men with evenly balanced feet, ankles, wrists and so on make the best lovers. In a study of 86 couples in their early twenties, the women claimed on average to have orgasms 60 per cent of the time. But that figure climbed to 75 per cent for those partnered to the most symmetric men, and dipped to about 30 per cent for women partnered to the most lopsided men. Not only that, the researchers found that the timing of the women’s orgasms varied in line with their partners’ symmetries. Couples were more likely to climax simultaneously, or close to it – possibly making conception more likely – if the man had a symmetrical body. Sadly for Cupid and his arrows, however, the data showed no link between orgasm frequency and love.
If this result stands the test of further scrutiny, one would have to conclude that the sculptors of ancient Athens were on to more than they imagined with their obsession for form and balance. But that ‘if’ is not to be taken lightly. What the researchers seem to be saying is that a few ounces of misplaced flesh on the body of a man can influence how many orgasms his partner has. Could men who advertise in lonely hearts columns benefit from extolling the pleasing symmetries of their elbows? Do women unconsciously weigh up the symmetries of the ankles and wrists of the men they date?
Commonsense suggests not, and the researchers would agree. “We don’t think women are looking at asymmetry in hand width,” says Thornhill. A more likely influence would be facial symmetry, but even that may not be the only – or indeed main – cue for attractiveness. “Symmetrical males may be more dominant and have the highest self-esteem and this could influence their attractiveness,” says Thornhill. So too could their physical size. In a study of volunteers in Britain, John Manning and his colleagues at the University of Liverpool recently discovered that heavier men also tend to be the most symmetrical men. “Perhaps symmetry is so difficult to assess you need to use something else, like body weight, to detect it,” comments Baker.
Moreover, even if the preference for symmetrical mates is real, the debate doesn’t end there. Another question must be thrashed out: why should symmetry be sexy? Why couldn’t it be our culture, all those glossy images of ideal (and mostly symmetrical) faces and bodies in fashion magazines, that has conditioned us so?
Critics of biological explanations of human behaviour might extract some mileage from the idea. But it cuts little ice with researchers such as Thornhill. Yes, they admit, make-up manuals are bursting with advice on how best to “correct” facial asymmetries; and yes, people with crooked smiles have acquired an undeserved reputation for villainy. But these cultural biases are surely the products, not the causes, of our preference for symmetry. Why otherwise would we share it with earwigs and swallows?
A far more productive theory, say adaptationists, is the idea that symmetry equates with fitness. In a world replete with parasites and other infectious agents that can disturb development in and out of the womb – indeed in a natural world that is itself imbued with asymmetry – only individuals with the best genes and food supplies will succeed in developing perfectly symmetrical bodies. And it’s these individuals who, in the unromantic terms of neo-Darwinism, make the most desirable mates. Mates with good genes. Mates with whom you might want to bond strongly in the bedroom.
Or so the argument goes. Not every biologist would express such confidence. Many preach nothing but caution and restraint when it comes to reading adaptive significance into human behaviour. “Even if symmetry is correlated with heritable fitness, that’s far from saying that the preference evolved to allow females to pick the fittest males,” says Kirkpatrick. Some even believe that nature’s love of symmetry may be a mere byproduct of the way nervous systems evolved.
The idea here is that symmetrical forms don’t necessarily equate with biological fitness; they may just happen to be easier for networks of neurons to recognise. Until recently, that was just speculation. But in November came some striking evidence from studies based on artificial neural networks designed to simulate biological visual systems. Such networks can be “offered” a set of “target images” and can learn to discriminate between them by a process resembling evolution by selection. The starting point is a population of visually naive networks. Round by round, networks that choose the correct target image – opting for a bird’s tail, say, rather than a random pattern – are allowed to replicate and mutate. The end result is a population of networks expert in recognising tails.
That much is expected. But what Rufus Johnstone, a zoologist at the University of Cambridge, found is more surprising: networks trained in this way respond most strongly to images of symmetrical tails even though the training process involves no bias in this direction. In otherwords, the preference appears to emerge spontaneously. And it does so, says Johnstone, because a symmetrical tail will always be the closest match to the average shape of a set of training images.
Suddenly, the idea that nature loves symmetry solely because it signals fitness begins to look simplistic. Maybe we prefer symmetrical bodies because symmetry is what our visual systems happen to respond to most strongly. Then again, maybe this visual bias only explains why the preference first evolved in primitive lifeforms, not why it is such a persistent theme in studies of sexual behaviour. Johnstone, for one, is sitting on the fence: “The preference for symmetry may have originated because symmetrical traits are easier to recognise, but perhaps the reason that the preference has not since been discarded is that symmetry is also a cue for fitness.”
It’s all very confusing, but the complexities don’t end there. Critics point out that despite three decades of research into fluctuating asymmetries, ideas about what causes them are still rather vague. “Fluctuating asymmetry is easy to measure, but in the end it’s a black box; it’s not all clear what it’s an indication of,” says Kirkpatrick. Animal research, it’s true, shows that body asymmetries tend to increase with inbreeding and the loss of the kind of genetic variety that is normally sustained by sexual reproduction. Yet nobody has yet pinpointed the genes involved. In lab studies, fruit flies exposed to toxins, parasites or extremes of temperature develop with imperfect symmetry; as do swallow nestlings infected with parasites. But again, the cellular and molecular reasons for this are unknown.
And if symmetry really is a badge worn by the biologically elite, what specific qualities does it signal? In the case of Japanese scorpion flies, the answer is longevity and hardiness (individuals with symmetric wings outlive their more lopsided cousins in seminatural enclosures). And in racehorses, one can point to a certain fleetness of foot. Last year, Manning and Louise Ockenden reported a link between symmetry and speed in British thoroughbred racehorses. In a survey of 73 animals, the more symmetrical horses tended to have the highest speed ratings, as set by the British Horseracing Board.
In humans, though, links between symmetry and biological fitness are harder to come by. Gregory Livshits and his colleagues at the University of Tel Aviv have found that mothers who suffer infectious diseases during pregnancy tend to give birth to infants with high levels of body asymmetry. These researchers also claim that asymmetrical infants tend to succumb more easily to medical problems such as heart disease later in life. Other results amount to little more than fascinating, if bizarre, correlations. Who knows what to make of the observation that people with asymmetric teeth tend to carry heavy loads of mouth-dwelling microorgansims; or that schizophrenics have asymmetrical fingerprints?
It is the missing pieces in this jigsaw that Thornhill and Møller are now trying to find. With Manuel Soler at the University of Granada in Spain, the researchers have found that women with evenly-matched breasts are on average more fertile than less evenly endowed women. And by no small degree, either. In a sample of some 50 American women, those with no children turned out to have the most asymmetric breasts – differing in circumference by as much as 30 per cent. Conversely, women with the most children turned out to have the most evenly matched breasts, differing in size by 5 per cent or less. Trends of this kind should never be taken at face value: reproduction and fertility depend on such a multitude of variables, cultural as well as biological. Nevertheless, the researchers claim to have ironed out any obvious biases in their data, due for example to age differences.
A more troubling question is why breast asymmetry should be bad news for fertility. Is it because women with asymmetric breasts find it harder to conceive? Because they find it harder to attract partners? Or because they are more committed to their careers? The researchers see reproductive biology as the key to the puzzle. They speculate that fertility and breast symmetry may both be sensitive to the destructive effects of hormonal imbalances, developmental disturbances in the womb or certain hereditary effects. If symmetrical breasts are sexier it may be because of this kind of biological link with reproduction, say the researchers. In other words, men who find women with symmetric breasts attractive may be unconsciously tuning in to a fertility signal.
Big is beautiful
Such ideas will remain moored to the wilder shores of speculation for some time yet. But what is clear is that of all human physical characteristics, it is breasts that display the most fluctuating asymmetry. Why, nobody knows, but Thornhill and Møller point to the fact that enlarged female breasts have appeared but recently in human evolutionary history. At some point since our ancestors diverged from other apes, natural selection lifted the nutritional and genetic constraints that previously kept female breasts small, allowing them to grow. But maybe the lifting of those constraints carries a price: the loss of such precise control over the way breasts develop.
Thornhill takes this theme a stage further to develop a still broader vision of the importance of symmetry in facial attractiveness. In animals, fluctuating asymmetries reach their zenith in secondary sexual characteristics, traits whose physical bulk and drain on resources are often out of all proportion to their functional value – horns, spurs, ornamental tails, enlarged canines. This is no accident, says Thornhill. Such traits may have evolved precisely because they are a drain on resources, precisely because they are difficult to sculpt into perfectly symmetrical shapes. The signal they give out is: “the owner of this trait has such fit genes that he can afford to squander vast resources advertising the fact”. An advertisement that wasn’t so costly or difficult to perfect wouldn’t be much good because any individual would be able to replicate it. It wouldn’t be an honest advertisement of fitness.
Beauty matters
This is why facial symmetry may be so important in humans, speculates Thornhill. Human males have unassuming canines, no horns and no ornamental tails. But they do have prominent jawlines. What’s more, those jawlines lengthen and broaden during puberty in response to increases in testosterone levels. The timing of these events could be significant, says Thornhill. Over the past few years it has become clear that testosterone spells bad news for the immune system. During puberty the hormone may interfere with the body’s ability to fight off infections. One outcome is the adolescent nightmare of rampant acne. But a second, less obvious one, suggests Thornhill, could be the sex appeal of large and square male jaws. Perhaps only males with the most vigorous immune systems can produce perfectly sculpted jaws in the face of immunity-damaging surges of testosterone.
The wider idea here is that secondary sexual characteristics such as the male jawline may function as honest advertisements of immunological fitness; because they develop under adverse conditions. A similar argument may apply to cheekbones in men and women, which also change shape during puberty, apparently in response to testosterone. Is it an accident that women with prominent cheekbones – Faye Dunaway, IsabelIa Adjani and so on – are so often hailed as beauties?
The adaptationists believe not. But they face a long struggle in persuading others to look at the ingredients of beauty through the same Darwinian prism. Even those who might support the notion that beauty symbolises a pukka immune system may question its contemporary relevance. In Western societies cleansed of the risk of all but a few parasitic infections, surely education, a healthy bank balance and a nice smile are more important than symmetrical cheekbones?
Such arguments miss the point, say adaptationists. The risk of parasitic infections may seem low now, but the opposite would have been true for our ancestors. To them, symmetrical features in a mate might have made all the difference between the life and death of their offspring. “Many psychologists would like to say that because of our civilisation we have been able to jump out of the gorilla suit,” says Møller. “But civilised human history is very brief compared to evolutionary history. We have a lot of luggage from those times.”